Ring lasers have been known since the early 1960's (see e.g. U.S. Pat. Nos. 3,382,759, 3,484,169, 3,486,130, 3,642,375, 3,714,607, 3,826,575, etc.). They suffer from two major drawbacks, i.e. "mode pulling" (see U.S. Pat. No. 3,382,759 col. 1, line 69) and loss of accuracy due to instability of the frequency of the laser. Mode pulling prevents the detection of low rates of rotation and frequency instability upsets the direct relationship between the measured beat frequency and the actual peripheral velocity of the ring. Many attempts, as illustrated in the above-cited references and others, have been made to solve these problems with varying degrees of success. So far, from the unclassified literature, it appears that these problems have not been solved adequately for ring lasers to go into widespread use despite the relatively low cost thereof compared to inertial systems, and a major demand therefor. Other methods of detecting rotation by the Sagnac effect have also been too crude or too cumbersome to achieve commercial success.